[s-cars] Can you teach an old fuel pump new tricks ???
forgied at direct.ca
Tue Jul 27 03:27:25 EDT 2004
S-gruppees: I just want to bounce this off you and see what you
think. Sean Douglas and I are now quite convinced that, based on his
fuel pressure readings, his (and my) "new" fuel pump(s) are not putting
out enough pressure to the fuel rail at full RS2 (plus) boost levels,
e.g. 22 to 28 psi. We have already noted that when the car is running
(and the alternator is running), the maximum voltage at the fuel pump is
only 12.5 V and it goes down as low as 11.8 V as the car gets warmer and
more boost is requested (by our heavy right feetses).
So, we have a brain wave: what if we double relay the fuel pump? Right
now, power goes from the battery/alternator to the central relay panel
position 6 under the drivers side dash. The ignition switch energizes
the relay and three contacts are closed. As far as I can tell by
examining the relay (I opened it up) and Bentley, two contacts seem to
be for the fuel pump and one seems to be for the O2 sensor heater. .
>From there the fuel pump power goes through Fuse No. 17 at the drivers
side end of the dash and then it runs ALL the way back to the fuel pump
which is located in the fuel tank which sits at the back of the trunk,
over the rear diff assembly. In going this round the world route, the
voltage drops in the order of 1.2 to 1.9 V depending on the load. I
just bet this doesn't help the fuel delivery and pressure situation.
Now as it happens, the fuel pump is only about 3.5 feet from the
battery, through the passenger seat bulkhead. So I am thinking, what if
we use the normal voltage from the existing relay to trigger a second
new relay located near the battery and then run a nice thick gauge hot
wire from the battery to this new relay and then to the pump. The
intent would be similar to adding relays to the headlights - cut out the
line losses and get more juice to the driven object, in this case, the
OEM fuel pump. Done properly, this should (hopefully) give us 13.7 ish
Volts at the fuel pump with the engine running. This has to make the
fuel pump spin a bit faster and, therefore, I suspect we may be able to
over come the existing fuel pressure deficits that Sean has estimated
and, therefore, get enough fuel to the engine under full boost that we
can start saying "Yeeeee HHhaaaaaah!!!" again (instead of tuning our
ears to the dreaded pinging death rattle).
Anysink wronk mit dis plan? Any fatal flaws in ze logic??
The other alternative, the Audi Sport fuel pump, is still suggested to
a) not have enough pressure and b) too much flow so this double relay
trick seems to be a potential good (and cheap) solution. The other is
too buy a case of OEM pumps and test each of them and pick the best two
of the litter. Unfortunately, I don't have that much time or money to
play that game.
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